1. Field of the Invention
The present invention is broadly concerned with catalysts for the abatement of pollutants, especially the abatement of NO.sub.x, HC and CO, particularly with their abatement in the exhaust gas of internal combustion engines. More specifically, the present invention is concerned with a novel composition which comprises a catalytic component and an oxygen storage component comprising an intimately mixed oxide of cerium and praseodymium.
2. Related Art
It is well-known in the art to utilize catalyst compositions, such as oxidation catalysts and those commonly referred to as three-way conversion catalysts ("TWC catalysts") to treat the exhaust gases of internal combustion engines. Oxidation catalysts promote the oxidation of unburned hydrocarbons ("HC") and carbon monoxide ("CO") in engine exhaust to H.sub.2 O and CO.sub.2. TWC catalysts promote such oxidation reactions as well as the substantially simultaneous reduction to N.sub.2 of nitrogen oxides ("NO.sub.x ") in the exhaust. It is well-known that successful functioning of the TWC catalyst to promote oxidation of HC and CO and substantially simultaneous reduction of NO.sub.x requires that the engine be operated at or close to stoichiometric air/fuel conditions.
It is also well-known in the art to provide such catalysts in the form of a refractory support material, such as a refractory metal oxide, e.g., activated alumina, on which is dispersed a catalytic metal component such as one or more platinum group metal components. The refractory metal oxide preferably has a high surface area to enhance the effectiveness of the catalytic metal component dispersed thereon. The catalytic component provided by the refractory support material having the catalytic metal component dispersed thereon is normally provided as a thin coating or "washcoat" adhered to the walls of a refractory substrate. The latter often takes the form of a body made from a suitable material such as cordierite, mullite or the like, which is formed to have a plurality of parallel, fine gas flow passages extending therethrough. Typically, there may be from about 150 to 450 or more such gas flow passages per square inch of end face area of the substrate.
A typical TWC catalyst will comprise one or more platinum group metals, typically including palladium, or palladium plus rhodium, or platinum plus rhodium, or platinum plus rhodium plus palladium, dispersed on an activated alumina to provide a washcoat coated on the gas flow passage walls of a suitable substrate. Optionally, a catalytic base metal component such as a transition metal of Group VIII of the Periodic Table of Elements, e.g., iron, nickel, manganese or cobalt, may be included in the composition.
Bulk ceria is known to be a useful additive for such catalyst compositions, especially TWC compositions in which the bulk ceria is believed to serve as an oxygen reservoir and is sometimes referred to as an oxygen storage component. It is believed that, with the engine operating at air-to-fuel ratios which fluctuate slightly above and below stoichiometric, the ceria supplies additional oxygen for the oxidation reaction during rich (relatively oxygen-deficient) periods of operation and takes up oxygen during lean (relatively oxygen-rich) periods of operation. Bulk ceria is not, however, immune to the problem of thermal degradation which affects other refractory metal oxides such as activated alumina. At elevated temperatures both activated alumina and bulk ceria suffer a reduction in their surface areas and this significantly reduces the effectiveness of the catalyst. It is known to stabilize refractory metal oxides such as alumina and ceria against such thermal degradation. One known technique is to impregnate into bulk alumina a solution of a soluble rare earth metal salt, e.g., a cerium salt such as cerium nitrate, and then calcine the impregnated alumina to provide a ceria-impregnated alumina to stabilize the alumina against thermal degradation. It is similarly known to stabilize bulk ceria against thermal degradation by impregnating it with a solution of a soluble aluminum salt such as aluminum nitrate, followed by calcination to provide an alumina-impregnated bulk ceria. While such impregnation techniques are effective to reduce the effects of thermal degradation, ceria is, nonetheless, subjected to degradation and marked reduction of the efficiency of the catalyst of which it is a part, not only by thermal degradation but also by poisoning of the catalyst by sulfur compounds, such as sulfur oxides which are engendered in the exhaust being treated from sulfur compounds contained in the fuel being burned.
U.S. Pat. No. 5,075,276, issued Dec. 24, 1991 to M. Ozawa et al, discloses a catalyst containing ceria as an oxygen storage component which is said to be useful for purification of exhaust gases. The Ozawa et al catalyst comprises a support substrate on which is disposed a washcoat comprising (a) a high surface area material which may be alumina or titanium oxide, (b) cerium oxide, (c) zirconium oxide and (d) at least one oxide of a rare earth element other than cerium and lanthanum. Ozawa et al's preferred atomic ratios per 100 cerium atoms are from 5 to 100 zirconium atoms and from 5 to 150 rare earth element atoms. Noble metals such as platinum, palladium, rhodium, etc., and base metals such as chromium, nickel, vanadium, copper, cobalt, manganese, etc., are exemplified as catalytic metals to be utilized on the Ozawa et al TWC catalysts (column 3, line 58 et seq.). Fifteen rare earth metals--including praseodymium--are listed at column 3, lines 33-43 as being suitable for item (d), although only yttrium, ytterbium, samarium and neodymium are exemplified. The Ozawa et al composition is said to suppress thermal degradation of the oxygen storage component (column 2, lines 5-9) which would otherwise occur unchecked, because of normal degradation of the ceria and consequent loss of surface area (column 1, lines 22-45).
Ozawa et al discusses the preparation of a composite oxide and/or a solid solution of (1) the oxide of the rare earth element other than cerium and lanthanum with (2) the cerium oxide, the zirconium oxide, or both. The composite oxide or solid solution is said to be obtained by either of two methods. One (column 3, line 65 et seq.) is by impregnating the catalyst layer (e.g., platinum on alumina) with three solutions of, respectively, a cerium salt, a zirconium salt and a salt of the rare earth metal, and then "burning" the impregnated catalyst layer at 600.degree. C. or higher. The other method (column 4, line 5 et seq.) comprises mixing the three oxide powders with alumina and "burning" the mixture at 800.degree. C. or higher. At column 4, line 12 et seq., Ozawa et al discloses that the cerium oxide and zirconium oxide may be present in the catalyst layer or may be loaded on the surface of the catalyst layer. The latter approach is said to improve the "catalyst property" remarkably.